A drop-down menu button
Inherits from Menu_
This is a button that when pushed pops up a menu (or hierarchy of menus).
Normally any mouse button will pop up a menu and it is lined up below the button as shown in the picture. However an Menu_Button may also control a pop-up menu. Not currently implemented in Lua FLTK. This is done by setting the type; see below.
The menu will also pop up in response to shortcuts indicated by putting a '&' character in the label.
Typing the shortcut of any of the menu items will cause callbacks exactly the same as when you pick the item with the mouse. The '&' character in menu item names are only looked at when the menu is popped up, however.